1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Description of the Related Art
In recent years, with the evolution of digital technology, nonvolatile memories have been developed, in which a large volume of data can be stored fast.
As such nonvolatile memories, a flash memory and a ferroelectric memory are known.
Of these, the flash memory has a floating gate buried in a gate insulating film of an insulated-gate field-effect transistor (IGFET). The flash memory accumulates charge representing information to be stored, in this floating gate, thus storing the information. However, in such a flash memory, tunnel current needs to be passed through the gate insulating film when information is written or deleted. For this reason, such a flash memory has a disadvantage that relatively high voltage is required.
On the other hand, the ferroelectric memory is also called a ferroelectric random access memory (FeRAM), and stores information by utilizing hysteresis characteristics of a ferroelectric film provided in a ferroelectric capacitor. The ferroelectric film is polarized in response to a voltage applied between upper and lower electrodes of the capacitor, and a spontaneous polarization remains therein even when the voltage is removed. When the polarity of the applied voltage is reversed, this spontaneous polarization is also reversed. Information is written to the ferroelectric film with the orientation of the spontaneous polarization corresponding to “1” or “0.” Voltage needed for this writing is lower than in flash memories. Moreover, the FeRAM has an advantage that faster writing can be performed than that in flash memories.
By utilizing this advantage, studies are being performed on applications of memory/logic mixed chips (SOC, system on chip), in which a logic circuit and an FeRAM are combined, to IC cards.
Different combinations of a lower electrode and the ferroelectric film, which constitute a ferroelectric capacitor, show different degrees of matching. Accordingly, ferroelectric characteristics of the ferroelectric capacitor greatly depend on the structure of the lower electrode.
For example, Japanese Unexamined Patent Application Publication No. 2005-159165 proposes that an IrO2/Ir/TiAlN/TiN stack structure is employed as a lower electrode.
Moreover, Japanese Unexamined Patent Application Publication No. 2000-91539 proposes that an IrO2 film, an Ir film, and a Pt film are formed in this order as a lower electrode.
Additionally, Japanese Unexamined Patent Application Publication No. 2004-95638 discloses that an Ir film and a Pt film are formed in this order as a lower electrode.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-164818 discloses that the fatigue resistance of PZT on a lower electrode is improved by forming a film stack including a Pt film and an Ir film as the lower electrode.
In addition, Japanese Unexamined Patent Application Publication No. 2003-298136 explains that it is preferable to employ a stack structure of Ir/Pt/Ir, Ir/Pt, Pt/Ir or the like as a lower electrode.